LM3S1968 Datasheet, PDF (241/709 Pages) List of Unclassifed Manufacturers – Microcontroller

Stellaris^® LM3S1968 Microcontroller
6.3.2
Clock Source
The Hibernation module must be clocked by an external source, even if the RTC feature is not used.
An external oscillator or crystal can be used for this purpose. To use a crystal, a 4.194304-MHz
crystal is connected to the XOSC0 and XOSC1 pins. This clock signal is divided by 128 internally to
produce the 32.768-kHz clock reference. For an alternate clock source, a 32.768-kHz oscillator can
be connected to the XOSC0 pin. See Figure 6-2 on page 241 and Figure 6-3 on page 242. Note that
these diagrams only show the connection to the Hibernation pins and not to the full system. See
“Hibernation Module” on page 661 for specific values.
The clock source is enabled by setting the CLK32EN bit of the HIBCTL register. The type of clock
source is selected by setting the CLKSEL bit to 0 for a 4.194304-MHz clock source, and to 1 for a
32.768-kHz clock source. If the bit is set to 0, the 4.194304-MHz input clock is divided by 128,
resulting in a 32.768-kHz clock source. If a crystal is used for the clock source, the software must
leave a delay of t_{XOSC_SETTLE} after setting the CLK32EN bit and before any other accesses to the
Hibernation module registers. The delay allows the crystal to power up and stabilize. If an oscillator
is used for the clock source, no delay is needed.
Figure 6-2. Clock Source Using Crystal
Input
Voltage
Regulator
or Switch
IN OUT
EN
Stellaris Microcontroller
VDD
XOSC0
X₁
R_L
XOSC1
C₁
C₂
R_PU
Open drain
external wake
up circuit
HIB
WAKE
VBAT
GND
3V
Battery
Note:
X₁ = Crystal frequency is f_{XOSC_XTAL}.
C_1,2 = Capacitor value derived from crystal vendor load capacitance specifications.
R_L = Load resistor is R_{XOSC_LOAD}.
R_PU = Pull-up resistor (1 M½).
See “Hibernation Module” on page 661 for specific parameter values.
July 15, 2014
241
Texas Instruments-Production Data